The invention pertains to a method for the contour measurement and/or deformation measurement of an object, as well as to a device for implementing a method of this type. The invention specifically pertains to a method for the interference measurement of an object. The object may consist, in particular, of a tire or a component of a composite material.
The tire may consist of a passenger car tire, a truck tire, an aircraft tire or a tire of another means of transportation. The method is suitable, in particular, for examining a non-mounted tire, i.e., a tire that is not mounted on a wheel rim.
Composite materials are used in numerous technical fields. They consist of two or more individual materials that are connected to one another. Such composites may be realized in the form of rubber-metal composites, polymers that are reinforced with carbon fibers, foamed polymers, honeycomb structures and structural components. Composites are also used for semi-finished products, for example, pipes, rods and plates. Electronic circuit boards usually are realized with composite materials. In addition, composite materials are used for aircraft parts, in particular, for the wing, the wing flaps, the landing flaps, the fuselage, spoilers, tail fins, control surface casings, rudder units, overhead compartment doors and the interior. In the automotive industry, composite materials are used, e.g., for energy absorbers, hoods, roofs, doors, dashboards, seat shells and motor vehicle frames. Composites are also used for containers and tanks that hold gases and liquids. In addition, they are utilized for sporting goods, for example, skis, snowboards and bicycles.
In the method and the device, the object or its surface is irradiated with light emitted by a radiation source. The light may consist of unstructured light. Unstructured light may be utilized, in particular, in the field of photogrammetry. In this case, a contour measurement and/or deformation measurement can be carried out with two or more cameras that produce images of the object or its surface under a certain angle. However, it is also possible to utilize structured light. Structured light is used, for example, in Moire methods, Graycode methods or triangulation methods. The object or its surface can be observed with a camera when structured light is used. However, it is also possible to irradiate the object or its surface with coherent or partially coherent light, particularly laser light. This type of light makes it possible, in particular, to carry out interferometric methods.
The light reflected by the object may consist of scattered light and is picked up by a camera. The camera contains an imaging sensor, in particular, a planiform sensor. A CCD sensor is particularly suitable for this purpose.
In the method according to the invention, the geometry or deformation of objects is determined with the aid of speckle images or interferometric images, in particular, shearography methods, holography methods, ESPI methods or white-light interferometry methods. These methods can be carried out with CCD sensors or other imaging sensors, namely with or without a phase image evaluation by means of a temporal or spatial phase shift. One or more radiation sources can be used for illuminating the object, particularly laser light sources. However, it is also possible to utilize the illumination devices employed in white-light interferometric methods, for example, thermal sources or LEDs.
When carrying out the method according to the invention, one or more images are produced in one or more different states of deformation of the object, wherein an average value is subsequently formed from said images. The various states of deformation of the object can be realized, in particular, by respectively subjecting the object to a different ambient pressure.
This is possible and advantageous, in particular, with respect to tires that are not mounted on a wheel rim. The tire can be introduced into a low-pressure chamber or a high-pressure chamber. One of the states of deformation is reached at an existing or deviating negative pressure or positive pressure, at which air enclosed in defective spots of the tire expands or contracts and produces local deformations of the tire that subsequently can be measured and detected.
The same method also can be carried out with other objects. In addition, it is possible to provide a low-pressure chamber or a high-pressure chamber that is composed of a plate of a transparent material and sealed relative to the object by means of a peripheral seal. A negative pressure or positive pressure can be adjusted in the sealed region between the object and the plate such that air inclusions are expanded or contracted. This causes corresponding deformations to occur at these defective spots of the composite material, and these deformations can be observed through the plate.
The various states of deformation of the object, however, can also be realized by subjecting the object to different temperatures, for example, heat that is generated, in particular, by means of radiation. This is particularly advantageous with components consisting of a composite material.
A differential deformation and/or its gradient can be calculated from different state images that respectively represent different states of deformation. The light source or light sources and the camera may be realized in the form of one unit that represents a measuring head.
A device for testing tires is known from DE 199 44 314 A1. This device comprises a positioning apparatus for the tire to be tested and a test apparatus, particularly a laser test apparatus. The test apparatus comprise several measuring heads, particularly laser measuring heads.
A method for illuminating objects for interferometric speckle shearography methods is known from DE 101 28 334. In this case, the object surface is simultaneously illuminated by one or more respectively coherent, dilated or planiform radiation sources from several illuminating directions with different sensitivity vectors. The illuminated partial regions of the object section to be examined may overlap one another completely or partially such that a uniform illumination can be achieved. The angle of incidence of the light beams emitted by the illumination source at the respective points on the surface of the object section to be examined can be chosen such that the respectively corresponding ideal angles of reflection are, referred to the local surface normal, not directly oriented into the imaging plane of the sensor and direct radiations are prevented.
EP 1 061 332 A2 discloses an electronic shearography method that is used for testing tires. In order to achieve an improved presentation of the shearography images, the different images are stored with a predetermined frequency and then displayed in the form of a sequential series. In this case, it is possible to display several image sequences that show different regions of the test object simultaneously.
U.S. Pat. No. 4,707,594 describes a method for testing tires, in which a tire that is not mounted on a wheel rim is introduced into a low-pressure chamber. The tire is tested by means of a holography method or a shearography method in this case. The tire is subjected to a sudden pressure change such that its surface progressively creeps for a comparatively long time. A series of successive images is produced during this time.
DE 28 06 845 C2 discloses a method for carrying out an interferometric examination of an object, in which an imaging system produces a first and a second image of the examined object that is illuminated with coherent light on a photographic medium in two different object states.
When carrying out the initially described method, it may occur that the dynamics of the sensor are not sufficiently high for obtaining sensible measuring values in the dark and in the bright image regions, particularly when examining uncooperative objects with dark and/or highly reflective surfaces. This problem may arise, in particular, when examining very shiny tires or composite materials.